The prior art will be explained hereinafter with reference to drawings. FIG. 6 is a cross-sectional view of a conventional example. In FIG. 6, at each side of the diaphragm 10 is attached each of fixed electrodes 15 and 20. One fixed electrode 15 comprises a first electrically conductive plate 12 arranged to confront to the diaphragm 10, an insulating plate 13 joined to this first electrically conductive plate 12, and a second electrically conductive plate 14 joined to this insulating plate 13, in which the first electrically conductive plate 12 is electrically connected with the second electrically conductive plate 14 through a conductor film 27 coated on the inner circumference of a pressure introducing hole 25. On the other hand, the fixed electrode 15 is provided with an annular support 21 separated by a circular groove 23 joined to the insulating plate 13 to surround the first electrically conductive plate 12, and this support 21 is joined to the diaphragm 10 through a glass junction member 11 having a predetermined thickness, and the first electrically conductive plate 12 is electrically insulated from the support 21. Incidentally, the support 21 may be any one of an insulator or a conductor. Further, the fixed electrode 15 is opened with the above-mentioned pressure introducing hole 25 for introducing pressure P1 into an air gap 29 formed between the diaphragm 10 and the fixed electrode 15. Another fixed electrode 20 comprises a first electrically conductive plate 17 arranged to confront to the diaphragm 10, an insulating plate 18 joined to this first electrically conductive plate 17, and a second electrically conductive plate 19 joined to this insulating plate 18, in which the first electrically conductive plate 17 is electrically connected with the second electrically conductive plate 19 through a conductor film 28 coated on the inner circumference of a pressure introducing hole 26. On the other hand, the fixed electrode 20 is provided with an annular support 22 being separated by a circular groove 24 joined to the insulating plate 18 to surround the first electrically conductive plate 17, and this support 22 is joined to the diaphragm 10 through a glass junction member 16 having a predetermined thickness, and the first electrically conductive plate 17 is electrically insulated from the support 22. Incidentally, the support 22 may be any one of an insulator or a conductor. Further, the fixed electrode 20 has the above-mentioned pressure introducing hole 26 for introducing pressure P2 into an air gap 30 formed between the diaphragm 10. Further reference numerals 31, 32 and 33 indicate capacitance output conductors respectively.
A first capacitor is formed by the diaphragm 10 and the fixed electrode 15, and a capacitance C1 of this capacitor is taken out through each of lead pins A and C. Also, in the same manner, a second capacitor is formed by the diaphragm 10 and the fixed electrode 20, and a capacitance C2 of this capacitor is taken out through each of lead pins B and C. Now, when each of pressures P1 and P2 acts on the diaphragm 10, the diaphragm 10 provides displacement depending on a differential pressure therebetween (P1.about.P2), each of the capacitances C1 and C2 changes depending on the displacement, and the differential pressure can be measured on the basis of this change. By the way, the differential pressure detector shown in FIG. 6 is accommodated in a housing sealed by two seal diaphragms not shown in the figure for receiving each of the pressures P1 and P2, and in this housing is enclosed a non-compressible fluid such as for example silicone oil for transmission of pressure. Namely, each of the air gaps 29 and 30 and each of the pressure introducing holes 25 and 26 are filled with silicone oil.
FIG. 7 is a schematic diagram of capacitance represented by the fixed electrode 15 of the left side shown in FIG. 6 formed in the conventional example, and FIG. 8 is an equivalent circuit diagram concerning the capacitance of the conventional example as shown in FIG. 6. Incidentally, with respect to each capacitance at the right side, the symbol is 2 instead of 1 at the left side with providing the same values for corresponding those at the left and right. In FIG. 7 and FIG. 8, capacitance C10, C20, C11, C12, C13 and C14 are given by EQU C10=Co/(1-.DELTA./d)=Ko.epsilon.r/(1-.DELTA./d) (1) EQU C20=Co/(1+.DELTA./d)=Ko.epsilon.r/(1+.DELTA./d) (2) EQU C11=K1.epsilon.r (3) EQU C12=K2.epsilon.r (4) EQU C13=K3.epsilon.c (5) EQU C14=K4.epsilon.c (6)
wherein
Co: common capacitance at the central portion when the differential pressure is zero, PA1 .DELTA.: displacement of the diaphragm on account of the differential pressure, PA1 d: air gap at the central portion when the differential pressure is zero, PA1 .epsilon.r: dielectric constant of silicone oil, PA1 .epsilon.c: dielectric constant of the insulating plate, PA1 Ko: constant determined by the shape of the central portion, PA1 K1, K2, K3, K4: constants determined by the shapes of each portion of C11, C12, C13, C14. PA1 .DELTA./d=0.2 PA1 Ko=10.555 PA1 K1=0.7218 PA1 K2=0.1197 PA1 K2 .epsilon.ro=0.323 (pF) PA1 K3 .epsilon.c=0.8498 (pF) PA1 capacitances are formed between a diaphragm as a movable electrode which is subjected to displacement depending on a differential pressure and fixed electrodes which are arranged at each side of the diaphragm respectively, on the basis of which said differential pressure is measured, and wherein PA1 each of said fixed electrodes is provided with PA1 a first electrically conductive plate closely adjoining and confronting to the surface of the central portion of said diaphragm; PA1 an annular support joined to the surface of a circumferential edge portion of said diaphragm separated from the outer circumferential surface of the first electrically conductive plate so as to surround it; PA1 a solid insulator of completely filling an annular confronting space between said first electrically conductive plate and the annular support; PA1 an insulating plate commonly joined to each surface of said annular support and said first electrically conductive plate at the reverse side to said diaphragm; and PA1 a second electrically conductive plate joined to the other surface of the insulating plate and electrically connected with said first electrically conductive plate, and PA1 a pressure introducing hole through the central portion.
Provided that the overall capacitance between each of the lead pins A and C is C1, and the overall capacitance between each of the lead pins B and C is C2, there are given: EQU C1=Ko.epsilon.r/(1-.DELTA./d)+K1.epsilon.r+K2.epsilon.r/(1+K2.epsilon.r/K3. epsilon.c)+K4.epsilon.c (7) EQU C2=Ko.epsilon.r/(1+.DELTA./d)+K1.epsilon.r+K2.epsilon.r/(1+K2.epsilon.r/K3. epsilon.c)+K4.epsilon.c (8)
When these C1 and C2 change in a differential dynamic manner, the following signal f which is proportional to the differential pressure can be obtained by means of a well-known arithmetic operation circuit: EQU f=(C1-C2)/(C1+C2-.beta.)=.DELTA./d=k(P1.about.P2) (9)
wherein EQU .beta.=2 [K1.epsilon.r+K2.epsilon.r/(1+K2.epsilon.r/K3.epsilon.c)+K4.epsilon.c, k: proportional constant.
However, when this detector is driven under a high static pressure so as to take out a signal proportional to the differential pressure, the dielectric constant .epsilon.r of the non-compressible fluid, for example, silicone oil changes depending on the static pressure, so that such a problem has caused that the span becomes narrow. This problem will be explained hereinafter in detail. According to experiments by the inventors, when silicone oil is used, 1.3% of a change in the dielectric constant is given for 100 kg/cm.sup.2 of a change in the static pressure, so that the dielectric constant .epsilon.r at the static pressure Ps can be represented as follows: EQU .epsilon.r=.epsilon.ro(1+0.013Ps/100) (10)
However, .epsilon.ro is the dielectric constant at Ps=0 (under atmospheric pressure). When the equation (10) is substituted for each of the equations (7) and (8) , and calculation is conducted using the following numerical values on the basis of the equation (9) , in the case of representation in which the axis of abscissa is the static pressure and the axis of ordinate is the span fluctuation (%) as shown by the representation with a broken line in FIG. 5, the characteristic of the span fluctuation against the static pressure Ps, that is the influence of the static pressure is considerably large such that the span fluctuation is -0.35% for the static pressure of 400 kg/cm.sup.2 In the meantime, the numerical values used for the calculation are as follows: